1. Field of the Invention
The present invention relates to a color image forming apparatus.
2. Description of the Related Art
Electrophotographic color image forming apparatuses have been widely used in the fields of small offices and general personal users. The color image forming apparatuses used in such fields are required to be more downsized, to have longer operating lives, and to further reduce costs than large color image forming apparatuses used in business or industrial purposes.
When such a color image forming apparatus is designed to meet the requirements of downsizing, longer operating life, and cost reduction, particularly a layout of functional parts needs to be examined in designing a photosensitive element serving as an image carrier and its peripheral parts, for example. For the downsizing of the photosensitive element, it is required to downsize a charging unit, a developing unit, a transfer unit, and a cleaning unit arranged adjacent to the photosensitive element as peripheral units in addition to the downsizing of the appropriate photosensitive element. However, it is difficult to downsize such peripheral parts because they are functional parts. The layout design has been elaborated to downsize the peripheral parts, but downsizing in such a way has come close to the limitation.
A lubricant applying member is one of the peripheral parts and applies lubricant on a surface of the photosensitive element. The lubricant applying member reduces abrasion of the photosensitive element caused by friction. Recently, rather than using the lubricant applying member, a lubricating component such as silicone oil is added to toner as an external additive to reduce friction on the surface of the photosensitive element. The use of such toner containing the lubricating component can eliminate the lubricant applying member, thereby achieving a low cost and further increasing flexibility of a layout of the functional parts arranged around the photosensitive element. As a result, the image forming apparatus is readily downsized.
In addition to the photosensitive element, other functional parts such as a developing roller have been advanced in downsizing, a longer operating life, and a cost reduction. As a recent trend, running costs have been lowered progressively by increasing durability of the functional parts including the developing roller in a process unit and thereby reducing frequency of replacement of the functional parts by a user.
With an extended operating life of the process unit, a space is newly needed in the process unit to store toner required in the increased operating life. This increases the size of the process unit, and thereby increasing the size of the image forming apparatus.
In view of such problems, a cartridge supplying technique has been proposed in which the amount of toner contained in the process unit is reduced and a toner cartridge compensates for a shortage. The cartridge supplying technique can achieve both of the downsizing and cost reduction of the image forming apparatus because a large amount of toner is not required to be contained in the cartridge; and a user replaces a used cartridge with a new cartridge each time toner is nearly exhausted.
The toner to which the lubricating component is added has increased adhering force between toner particles, thereby causing fluidity of the toner to deteriorate. In addition, the toner receives stress due to contact friction when the toner sequentially passes through a supply roller, a developing roller, a regulating blade, and a photosensitive element while making contact with them in the printing operation. The stress due to the friction causes: the lubricative external additive of the toner to come off; the lubricative external additive to be buried in the surfaces of the toner particles; or the toner particles to be broken or deformed.
Accordingly, the stress not only reduces the charged potential of the toner but also causes the toner to be readily broken with the deterioration of the fluidity of the toner. As a result, a transfer hollow phenomenon (what is called a “vermiculation phenomenon”) may occur. In the transfer hollow phenomenon, toner is not fully transferred from the photosensitive element to an intermediate transfer belt in a primary transfer area between a photosensitive drum and the intermediate transfer belt, resulting in central areas of images (particularly, central areas of lines or characters) on the intermediate transfer belt being missing.
Japanese Patent Application Laid-open No. 2003-57916 discloses that a circumferential speed of an intermediate transfer belt is set faster (0.1%) or slower (0.1%) than the circumferential speed of any of a plurality of photosensitive drums in transferring operation for the purpose of preventing the transfer hollow.
Japanese Patent Application Laid-open No. 2007-304405 discloses that a linear velocity difference between an intermediate transfer belt and a photosensitive drum is set in such a manner that the linear velocity difference is set smaller for a photosensitive drum for black disposed at the most downstream position and for a photosensitive drum for color disposed at the most upstream position, and set larger for photosensitive drums for other colors arranged between the most upstream position and the most downstream position.
Japanese Patent Application Laid-open No. 2003-57916 aims to prevent the transfer hollow caused by agglomeration of toner on the photosensitive element as a result of the toner on the photosensitive element being compressed in a primary transfer nip. It has been found that this method, however, is not an effective solution for preventing the transfer hollow because this method may promote the occurrence of the transfer hollow because of the toner having poor fluidity due to the added lubricating component.
The transfer hollow is the direct result of the agglomeration of toner, which is what is called a “packing”, caused by a pressure applied to a toner layer on the photosensitive drum in the transferring operation. The toner layer before being transferred forms a layered structure having appropriate gaps between toner particles. The soft layer is pressed by a force applied thereto when pressure is applied in the transfer operation. In the pressing, an edge portion of the toner layer crumbles to surrounding areas including no toner layer to distribute the pressure while a central portion of the toner layer is pressed and agglutinated (packed) because the central portion hardly moves to surrounding areas and cannot distribute the applied pressure.
The toner thus packed increases sticking force to the surface of the photosensitive drum and cannot be readily moved (transferred) to the intermediate transfer belt even though a predetermined transfer electric field is applied. As a result, the “transfer hollow” occurs. The toner having poor fluidity due to the added lubricating component readily causes the “transfer hollow” particularly because the toner is readily agglutinated.
When the linear velocity of the intermediate transfer belt is set faster than the photosensitive drum as the method for preventing the transfer hollow as disclosed in Japanese Patent Application Laid-open No. 2003-57916, the toner on the photosensitive drum receives a shearing force in the moving direction of the intermediate transfer belt when passing through the primary transfer nip, the number of toner particles per unit area is reduced when the toner is transferred from the drum to the belt, and thus the toner thickness is effectively reduced in the nip portion. As a result, generally, the packing hardly occurs.
However, it has been found that primary transfer efficiency and graininess of dots deteriorate when excess liner velocity difference is set between the photosensitive drum and the intermediate transfer belt, on the base of the results of experiments performed by the applicant. That is, the prevention of the transfer hollow and the increase in primary transfer efficiency or improvement of graininess of dots are in the relation of trade-off.
There is a need to enable the occurrence of the transfer hollow to be reliably prevented for long period of time even when the toner is used that has low fluidity and high adhering force between toner particles due to the added lubricating component, and to achieve both of the prevention of transfer hollow and the downsizing, the longer operating life, and the cost reduction of the image forming apparatus in a good balanced manner.